1. Field of the Invention
The present invention relates to systems for applying and sealing plastic strap around an object, and in particular to systems for forming a heat-sealed joint between overlapping portions of the plastic strap.
2. Description of the Prior Art
A number of prior techniques have been used to heat seal two portions of plastic strapping. These techniques have included the insertion of a hot knife between the overlapped strap portions to melt the facing strap surfaces; melting the facing strap surfaces by friction; ultrasonic welding of the strap portions; dielectric welding of the strap portions; the use of adhesives to join the strap portions together; and the use of radiant energy absorption by an opaque layer on one of the strap portions to melt the facing strap surfaces. The present invention relates to this latter technique of radiant energy strap sealing.
The heat sealing of plastic straps by the use of radiant energy is disclosed in United Kingdom published application U.S. Pat. No. 2,103,147, and entails the use of plastic strapping which is a composite of a relatively thick translucent layer and a relatively thin opaque layer. The strap portions to be joined are overlapped and are pressed against the window of a tungsten-halogen lamp assembly so that the opaque layer of the lamp-side strap portion is in pressure contact with the other strap portion. The overlapping strap portions are then irradiated through the window with a high intensity transmission of radiant energy, primarily in the visible wavelengths. This energy passes through the translucent layer of the lamp-side strap portion with little or no absorption, and is preferentially absorbed by the opaque layer, which melts and fuses to the other strap portion to form a heat sealed joint therewith on cooling after the lamp has been switched off.
While this prior system forms an effective, high-strength joint between the strap portions, it has been found to have certain practical operating disadvantages as a result of the necessity of clamping the strap portions against the lamp window. This clamping is necessary in order to ensure firm, uniform contact between the overlapping strap portions and to ensure that the strap portions are held stationary during the melting and fusing of the opaque layer, otherwise uneven and imperfect joints will result. But this clamping arrangement results in conduction of heat from the lamp window to the straps, since the lamp window is not perfectly transparent to radiant energy. The more rapid the operation of the sealing apparatus, the more the heat is absorbed by the window. This retained heat is, in turn, conducted from the window to the adjacent strap portions in succeeding cycles.
This tends to defeat one of the prime intended advantages of the radiant energy technique, i.e., the selective heating of only a thin opaque layer at the interface of the overlapping strap portions, without significant heat absorption by the remainder of the strap. Selective heating of only a thin surface layer substantially eliminates thermal degradation and resultant weakening of the remainder of the strap thickness. But conduction of heat from the window back into the strap tends to heat and thermally degrade the translucent portion of the strap.
Furthermore, the heat conducted into the strap from the window may be sufficient to reach the opaque layer and thereby effectively lower the heat contribution necessary from the lamp in order to attain optimal fusion of the strap portions. Thus, since the lamp is typically operated for a predetermined time period during each cycle, there will result overheating of the opaque layer and resultant heat conduction into and melting of the adjacent portions of the translucent layer. Indeed, it has been found that the lamp window may become so hot that it can start to melt the strap without any contribution at all from the lamp.